A Plumber Who Specializes in Cleaning Plugged Sewer Lines Is

A plumber who specializes in cleaning plugged sewer lines is typically paid a higher wage than a plumber who works on installing water systems in new residential housing.This can be partially explained by the fact that

A) plumbers who work with sewer lines require years of specialized training.
B) plumbing jobs that involve sewer lines are likely to have an element of personal satisfaction.
C) plumbers who work with sewer lines are more likely to work a standard shift.
D) plumbers who work with sewer lines are likely to be perceived as having a "dirty" job.